As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Typically, an upper portion of the plastic container defines an opening. This upper portion is commonly referred to as a finish and includes some means for engaging a cap or closure to close off the opening. In the traditional injection-stretch blow molding process, the finish remains substantially in its injection molded state while the container body is formed below the finish. The finish may include at least one thread extending radially outwardly around an annular sidewall defining a thread profile. In one application, a closure member or cap may define a complementary thread, or threads, that are adapted to cooperatively mate with the threads of the finish.
In some applications, plastic containers must withstand extreme temperatures and pressures, while providing an improved heat resistance and an ability to withstand vacuum forces at a reduced weight as compared to traditional designs. In other examples, conventional plastic containers that are cylindrical in profile, do not include a sidewall design with adequate vertical load bearing and dent resistance capabilities and properties.
Thus, there is a need for a plastic container design having a sidewall capable of achieving optimal performance with regard to vacuum absorption, top load strength capabilities and dent resistance.